1. Field of the Invention
The disclosures discussed herein relate to a developing device and an image forming apparatus, and more specifically relate to toner supply control for use in binary developer (i.e., two-component developer).
2. Description of the Related Art
In an image forming apparatus such as a copier, a printer, and a facsimile machine, a developing device generally conducts a process for visualizing an electrostatic latent image formed on a photoreceptor serving as a latent image carrier, and a visualized image is transferred onto a sheet or the like, thereby outputting a recording result.
As a developer used for the developing process, there is a two-component system (or a binary system) developer formed of a mixture of a toner and a carrier, other than a one-component system developer formed of either magnetic or nonmagnetic toner alone. The binary developer may be configured to be ready to be electrostatically attracted to the electrostatic latent image on the photoreceptor by charging the toner with a frictional electrification action generated at the time of stirring the mixture.
When the developing process is conducted, it may be necessary to optimize the amount of toner attracted to the electrostatic latent image on the photoreceptor for improving image quality. In order to optimize the amount of toner attracted to the electrostatic latent image, it may be necessary to manage density of toner contained in the two-component system developer. Specifically, it may be important to match a value (i.e., a detected value or a current value) of toner density and a target value of the toner density. In order to match the detected value of the toner density in the two-component system developer and the target value of the toner density inside the developing device, additional toner is supplied to the two-component system developer when the toner density is lowered. However, it may be difficult to accurately detect the amount of toner actually supplied to the two-component system developer corresponding to a set toner supply amount because the toner is formed of powder. Specifically, when there is environmental fluctuation such as temperature fluctuation, humidity fluctuation, or a setting changed by a user, the toner supply amount may need to be adjusted according to such fluctuation or the like.
In the related-art technologies, a toner supply control method is widely used. In the toner supply control method, a value of toner density is detected by a sensor, and the amount of toner to be supplied is controlled based on the detected result (i.e., the detected value). In this case, the toner density may be adjusted not only by increasing the toner density but also by decreasing the toner density. It may be possible to control the supply of toner to improve the toner density when the toner density is lowered in a design phase; however, it may be difficult to control consumption of toner in the design phase, which is often left under the usage conditions of the user.
For example, when a target value of the toner density may need to be changed toward a low toner density due to change in an optimal condition, the toner that is continuously consumed until the toner density reaches the set low toner density (i.e., the target value of the toner density) is not assumed in the design phase. Accordingly, the user may need to wait until the detected value of the toner density is lowered for the continuation of executing image formation. Accordingly, in the related toner supply control that specifically focuses on compensating the toner density reduction, it may be difficult to change the toner supply amount to the optimal toner density including changing the toner supply amount to the low toner density. Specifically, when the (setting of) toner density is changed to the low toner density, the user may need to wait until the toner is continuously consumed by forming images such that the toner density becomes low. This may result in an unstable image quality. Hence, images may be formed with undesired density, or the quality of the formed images may deteriorate.
As an example of a configuration to implement the above toner supply control method, Japanese Laid-open Patent Publication No. 2010-091785 (hereinafter referred to as “Patent Document 1”) discloses a configuration including a sensor to detect density of an image, where excess or deficiency of toner contained in developer is determined according to a change in the image density, and a toner supply amount is controlled based on the determined result. Further, in order to prevent delay in supplying toner or excessive toner supply when the toner is supplied, Japanese Laid-open Patent Publication No. 2006-171023 (hereinafter called “Patent Document 2”) discloses an example of a configuration in which a toner supply amount is computed by multiplying a toner supply rate computed based on image density by a predetermined coefficient.